Conventionally, magnetic recording medium is composed of a support or carrier sheet or strip (referred to as carrier strip hereinafter) made of a material such as a polyester film and a magnetic layer formed on the carrier strip by way of coating a magnetic powder with an adhesive synthetic resin. However in conventional recording medium, said magnetic layer is readily electrified because of its high surface resistivity, resulting in either generation of noise due to discharge of the electricity, or drop out of signal by dust attracted to the recording medium when reproduced.
In order to decrease the surface resistivity of the magnetic recording medium, it is known
(a) to add carbon black into the magnetic layer
(b) to provide conductive layer including carbon black between the carrier strip and the magnetic layer.
However, according to method (a), the density of the magnetic powder in the magnetic layer is lowered by the addition of carbon black of relatively particle size, and also the sensitivity of the magnetic recording medium tends to be lowered. In addition, the surface of the magnetic layer becomes relatively rough, thereby resulting in increasing of the output fluctuation and noise of the magnetic recording medium when it is reproduced.
The method of (b), which is disclosed in U.S. Pat. No. 3,440,091 serves to avoid problem of decrease of density of magnetic powder dispersed in the magnetic layer.
However, according to this method, in order to decrease the surface resistivity of the magnetic layer to 10.sup.10 .OMEGA. or more, preferably to 10.sup.9 .OMEGA. for preventing electrification of the recording medium, it is necessary to add carbon black in the conductive layer in an amount of more than 40% by weight. However, such a large amount of carbon black causes the surface of the conductive layer to be relatively rough, whereby the surface of the magnetic layer is badly affected, thereby causing the output uniformity of the recording medium to become worse.
In addition, as the amount of carbon black added in the conductive layer increases, namely, the amount of the binder agent decreases, the adhesiveness between the conductive layer and the carrier sheet may be weaken, the then a part of the magnetic layer tends to separate from the conductive layer during the scanning of the recording medium by a recording head, thereby resulting in a drop out of the signal.